This invention relates to drilling and cementing boreholes.
The drilling of boreholes is generally carried out using a rotary drilling process. The rotary drilling of a borehole is accomplished by rotating a drill string having a drill pipe and a drill bit at its lower end. Weight is applied to the drill bit while rotating to create a borehole into the earth. The drill string is hollow and sections are added to drill string to increase its length as the borehole is deepened. This rotary drilling process creates significant amounts of friction which produces heat along with fragments of the strata being penetrated. The fragments of the strata must be removed from the borehole and the drill bit must be cooled to extend its useful life. Both of these necessities are accomplished by the circulation of a drilling fluid down through the drill string and up to the surface between the drill string and the wall of the borehole.
Once the borehole has been drilled to the desired depth, it may be desirable to isolate the separate areas, zones or formations transversed by the borehole. For extraction of fluids from formations, a conduit (casing) must be inserted into the borehole extending from the surface downward, and liners may be hung inside the casing.
At this point it becomes necessary to fill the annulus between the casing and the borehole wall or between the liner and casing with a material which will seal the annulus and provide structural support for the casing or liner. This is commonly referred to as primary cementing.
In order to obtain a good cementing job, it is necessary for the cement slurry to displace substantially all of the drilling mud from the annulus. A reduced displacement efficiency arises from the fact that drilling fluids and cements are usually incompatible.
Non-displaced mud (mud still left in the borehole after cementing) and mud filter cake are major causes of unsatisfactory cement performance. Since the non-displaced mud and mud filter cake do not set or bond to the casing, the borehole wall or the set cement itself, the mud and filter cake do not support the casing properly and later can allow annular gas or liquid migration.
Wells frequently are drilled with water-base muds which contain oil as the internal emulsified phase. These muds are generally referred to as emulsion muds or emulsion drilling fluids. These emulsion muds are used, for example, to lower drilling torque and drag, and to prevent bit balling. The presence of the oil in the wellbore, however, can even further reduce the displacement efficiency and subsequent cementing with a conventional water-base Portland cement slurry.
The drilling industry has sought to overcome the above problems by using a variety of techniques to displace the drilling fluid with cement, e.g., turbulent flow, casing movement (reciprocation/rotation), casing equipment (centralizers, flow diverters, mud cake scratchers), and special spacers and wash fluids, but these have had limited success. When a good cementing job is not obtained, it may be necessary to perforate the casing and squeeze cement under high pressure through the perforations into the annulus and try to fill the zones that were not properly cemented initially. Frequently, this squeeze cementing is not successful, and such failures may eventually lead to abandoning the hole.
One of the major objectives of a primary cementing is to obtain good zonal isolation in the annulus of the well. Effective zonal isolation is achieved by sealing the cement and borehole wall. The interface of the cement and borehole wall is usually an interface between the cement and drilling fluid filter cake which is the source of many cementing problems. Good zonal isolation can only be achieved if the filter cake hardens, permanently bonds to the formation face and the cement, and provides hydraulic sealing.
Cementing of the annulus between an oil well casing and the surrounding borehole has been practiced for over 90 years. Long ago, it was recognized that it would be advantageous to solidify drilling fluid in the annulus so as to reduce the cost of the cementing of the casings. Over the decades, various attempts have been made along this line as evidenced by the disclosures in Williams et al., U.S. Pat. No. 2,649,160 (Aug. 18, 1953), and Miller et al, U.S. Pat. No. 3,887,009 (Jun. 3, 1975). However, such techniques, while presumably operable, have failed to achieve commercialization even after the passage of many years. Another attempt in this direction is described in Wyant et al., U.S. Pat. No. 3,499,491 (Mar. 10, 1970), wherein Portland cement is used in an aqueous drilling fluid to produce a mud concrete. Portland cement, however, is very sensitive to the water/solids ratio. Even under ideal conditions, only small increases in the amount of solids results in a very viscous mass. On the other hand, only a slight decrease in the amount of solids results in a composition which sets up to give a very weak structure. These problems are amplified when attempting to use Portland cement in a drilling mud. Thus, while this technique has been used commercially, it has serious drawbacks.
Slag, broadly, as a cementitious material is shown in Tragessar, U.S. Pat. No. 3,557,876 (Jan. 26, 1971).